1. Field of the Invention
The present invention relates to a semiconductor device including an insulating film made by high frequency sputtering method using silicon as a target and a manufacturing method of the same. More particularly, the present invention relates to a gate structure of an insulated gate field effect transistor, typified by a thin film transistor (TFT).
2. Description of the Related Art
It is a known technique is that a TFT formed on a substrate such as a glass substrate, a liquid crystal or an electroluminescence (EL) material and a pixel combined with an element thereof is arranged in a matrix to make a display showing characters or images. A TFT, wherein a channel portion overlapped with a gate electrode is formed with a polycrystalline silicon film, can operate a logic circuit or the like at a suitable frequency.
A polycrystalline silicon film for this purpose is an amorphous silicon film crystallized by a laser annealing method or the like. Of course, the electrical property and reliability depend not only on the quality of the polycrystalline silicon film, but also on the quality of a gate insulating film or a base insulating film contacting with the polycrystalline silicon film.
In a MOS transistor formed on a single crystalline silicon substrate a high quality gate insulating film can be formed, utilizing a thermal oxidation technique effectively. However, in making a liquid crystal display using a glass whose distortion point is 700° C. or less as a substrate, a technique to deposit films using a chemical or physical phenomenon is adopted. This is different point from the manufacturing technique of silicon integrated circuit including a heat process at 700° C. or more.
Focusing on the gate insulating film of a transistor, a desired property is that the gate insulating film is a dense film without holes and pinholes, including no mobile ions. Further, the gate insulating film has low defect level density due to its fineness and has less leak current such as gate leak.
However, insulating films such as silicon oxide and silicon nitride that are deposited using chemical or physical reaction by conventional plasma CVD or sputtering method have a disadvantage such as defect and pinholes resulted from being easily damaged by charged particles, fixed charge or high interface state level. Also, it lacks in a thermal stability due to several atoms percent of hydrogen contained in the insulating film. And it is also a cause of a characteristic deterioration such as fluctuation in a threshold voltage or increase of a gate leak current.
On the other hand, as an integrated circuit made with TFTs becomes more micro fabricated, much thinner film of a gate insulating film is more important based on scaling law. It has been impossible for a silicon oxide film or a silicon nitride film deposited at 400° C. or less to obtain a desired characteristic with reducing fixed charges and interface state level densities, as contrasted to a pure silicon oxide film that is oxidized at 900° C. or more.